(1) Field of the Invention
The present invention relates to a process of producing Maddrell salt, i.e. a long-chain, high-molecular weight sodium polyphosphate and more particularly to a simple and highly advantageous process of producing said salt in a form which is substantially free of water-soluble by-products and to such a substantially water-insoluble Maddrell salt.
(2) Description of the Prior Art
Heretofore, Maddrell salt has been produced by heating sodium orthophosphate or disodium pyrophosphate at a temperature between 250.degree. C. and 350.degree. C. However, only little attention has been paid to its preparation since due to its insolubility in water no utility had been found therefor. This is also the reason why no quantitative investigations regarding the amount and the type of its water-soluble components-- which always are obtained when producing the salt according to the methods known and used heretofore-- were carried out. The water-soluble components of said salt were simply eluted with water to produce a salt useful for scientific investigations.
However, a few years ago Maddrell salt has been found to be useful on a large scale as polishing and cleaning agent in toothpastes. For such a use the salt should have as low a water-soluble component as possible. When dehydrating mono-sodium orthophosphate in the known manner to Maddrell salt, there are always formed thereby the water-soluble compounds sodium trimetaphosphate and di-sodium pyrophosphate in an amount depending almost exclusively on the manner in which the temperature is adjusted during dehydration. Usually the amount of water-soluble components exceeds 5 %. Said water-soluble compounds have an extremely negative effect upon the consistency, effectiveness, and stability on storage of the toothpaste containing Maddrell salt.
In the meantime, several new processes of producing Maddrell salt have become known. Thus a process for producing Maddrell salt with up to 4 % of water-soluble components is described in U.S. Pat. No. 2,356,799. According to this process, monosodium orthophosphate must be shaped to pellets in a first reaction step. Said pellets are then converted into Maddrell salt by heating to a temperature between 300.degree. C. and 460.degree. C. This mode of operation, however, is quite complicated and expensive because an additional process step of producing pellets is required.
Furthermore, it is stated in "Journal American Chemical Society" vol. 81, p. 79 (1959) that neither pelletizing nor very fine comminution of monosodium orthophosphate or considerably prolonged thermal treatment are able to reduce the water-soluble portion in the final product below 5 % calculated for total substance.
J. R. Van Wazer in his book on "Phosphorus and its compounds" vol. 1, p. 668, has disclosed that, when converting monosodium orthophosphate to Maddrell salt at a temperature of 400.degree. C., water vapor is formed by splitting off the water of constitution. Such formation of water vapor may even have a negative effect upon the formation of Maddrell salt.
German Published Application No. 1,667,569 describes a process of producing Maddrell salt from monosodium orthophosphate by heating the latter to 450.degree. C. and removing by suction the water vapor formed due to the condensation reaction. The water vapor partial pressure is maintained during this reaction between 50 Torr. and 450 Torr.
As described in "Zeitschrift Anorg. Allg. Chemie", vol. 258, p. 52 (1949) and in "Analytical Chemistry", vol. 30, p. 1101 to 1110 (1958), Maddrell salt is produced by heating monosodium orthophosphate to a temperature of 350.degree. C. or, respectively, 380.degree. C. within a period of time between about 50 hours and about 168 hours. Thereby, a product is obtained which is contaminated to a considerable extent. It must be purified by washing out the by-products.
Another process of producing Maddrell salt is described in German Pat. No. 2,161,600. Said process consists in dehydrating monosodium orthophosphate at a temperature between about 300.degree. C. and about 380.degree. C. in the presence of water vapor. According to this process dehydration is carried out in a saturated water vapor atmosphere. This process is also rather complicated since it is necessary that a predetermined water vapor partial pressure is maintained during dehydration.